The invention relates to a system for controlling the traction power of a utility vehicle having an infinitely variable transmission. It is particularly suited to a vehicle with an infinitely variable transmission in which power is split between hydrostatic and mechanical branches.
Vehicles which lave an infinitely variable, hydrostatic-mechanical transmission can be accelerated from standstill to a maximum speed in both directions of travel by adjusting the transmission and therefore do not require a conventional clutch between the transmission and the engine. However, in certain situations, e.g. for changing the speed of an additional mechanical transmission offering several speed ranges, it is necessary to reduce the traction power built up at a specific speed without at the same time reducing the speed. A device which meets this requirement is described in DE 195 23 963 A1. So that the operator controls of this vehicle and their layout as well as the vehicle's response to their use conform as far as possible to that of a utility vehicle with a conventional gear shift system, the traction power is controlled by a pedal, which, because of the way it operates, will be referred to as a clutch pedal hereafter. Depending on the degree to which the clutch pedal is opened, orifice plates associated with respective directions of travel and controlled by the clutch pedal each affect the closing power of a valve, the latter being arranged one after the other in a hydraulic line extending between the two sides of the hydraulic motor forming part of the hydrostatic branch of the transmission. Operating the clutch pedal causes the valves to open in proportion thereto, which leaks pressure to a greater or lesser degree between the two sides of the motor. This causes a reduction in the available traction power in proportion to the reduction in pressure. Pressure will be leaked in different directions according to whether the motor is operating in forward or reverse modes.
In a high-power utility vehicle, the known control device has lived up to the expectations placed on it. However, when it comes to substantially lower-power vehicles, it tends to lack sensitivity in attenuating the transmitted traction power in the lower power range. This inadequate sensitivity causes a perceptibly rough driving behaviour during "meshing", making it more difficult to carry out front loading work, for example.
One objective of the invention is to provide a device of the type outlined above, which will permit the transmitted traction power to be sensitively proportioned irrespective of the vehicle drive output.